In a wireless communication system such as CDMA (Code Division Multiple Access) or WiMAX (Worldwide Interoperability for Microwave Access), the realization of time synchronization is sought between base stations with error no greater than microseconds. As a result, time acquisition is carried out using GPS (Global Positioning System) in devices that make up this type of wireless communication system (See Patent Document 1).
However, in devices that carry out time acquisition using GPS, various factors hinder the acquisition of accurate time. For example, breakdowns due to causes such as the deterioration of a device over time or due to damage to an antenna due to lightning. A state in which a GPS signal cannot be received due to this type of damage may continue for a short time or for a long time period.
For example, in a state in which the GPS satellite cannot be picked up, a synchronization device having a configuration in which an outside oscillator is controlled by a Phase Locked Loop (PLL) circuit that takes as input a timing signal that is based on a GPS time signal maintains time accuracy by relying on only the degree of the frequency stability of the outside oscillator. In a state in which a highly stable device such as a double-oven OCXO (Oven-Controlled Crystal Oscillator) is selected as the outside oscillator, the frequency stability is on the order of 10−10. With this level of stability, a shift of approximately 4.3 μsec occurs over 24 hours. In actuality, apart from the stability of the oscillator itself, the stability of a nearby power supply and the control voltage also affect the stability and the divergence in time may therefore be an even greater value.